Diablo 4 has an online 'shared open world' and a non-linear campaign

Diablo 4's darker direction, as seen in its announcement trailer, isn't the only thing that separates it from Diablo 3. Campaign details spilled out in the Diablo 4 Revealed panel following the opening ceremony, with the biggest point of interest being Diablo 4's new "shared open world" structure.  Rather than follow the series' usual straightforward level design, Diablo 4 will feature five big, contiguous regions—with a day-night cycle and dynamic weather system—the player can conceivably wander at their own discretion, assuming they have the gear to cut it. More important is the "shared" part of that description. We've yet to hear how many players will be wandering Diablo 4's open world at once, but the point is they'll be wandering with you.

Yep, Diablo is basically an MMO now.  Towns act as social hubs for chatting and grouping up, PvP zones act as seamless ganking areas, and world events spawn big monsters that strangers and teammates can take down together. The world is also apparently big enough to justify the addition of mounts. We'll be able to suit up our horses in armor, perform powerful dismount abilities, and explore the open world at a brisk clip.